Transmission of data via optical links, in general, is not few. For years, devices have employed optical, and particularly infrared, transmitters and receivers for allowing line-of-sight remote control of the devices. These optical remote controls have proven popular with consumers because they free the consumers from having to spend energy travelling to a device to control the device.
Most typically, an optical remote control transmitter comprises a power source, such as a battery, that is coupled to a keypad, a carrier wave generation circuit and a coding circuit. Depending upon which of the keys of the keypad are depressed, the coding circuit produces a code that modulates the carrier wave produced by the carrier wave generator. The modulated carrier wave energizes one or more infra-red light emitting diodes ("LEDs") to produce an optical signal that emanates from the transmitter. In almost all remote control transmitters the optical signal spreads in a conical pattern from the transmitter, and thus signal strength per unit area decreases as the square of the distance from the transmitter to the receiver. The amount of energy contained in this beam is limited by the capabilities of the battery and the efficiency of the LEDs. The signal strength within the beam at the receiver is directly proportional to the sensitive area of the detector element which intercepts the beam and inversely proportional to the distance between the transmitter and the receiver.
On the remote control receiver side, it is typical to provide a PIN-type photodiode as the receiving element. Such photodiodes lave a good response to infrared radiation because of their thicker depletion region, a very good high frequency response and low intrinsic capacitance, so that the optical signal is transformed into an electrical signal without serious degradation. However, because photodiodes have a very small detector area, usually less than 0.1 inches square, they intercept very little of the energy radiated by the transmitter. As a result, the output signal they produce is extremely small and necessitates the use of special high gain, low noise preamplifiers before a signal of usable amplitude is obtained. Photodiode receiver circuits are also sensitive to extraneous light sources that produce electrical noise in the detector output. Strong light sources saturate the detector circuit so that no usable signal can be detected.
As can be seen, the remote control transmitter must be fairly close to a photodiode-based remote control receiver before the photodiode can generate an electrical signal of usable strength. In general, the transmitter must be within 30 feet of the receiver for the photodiode to generate a satisfactory signal. Furthermore, special baffles and optical filters are mandatory if the system is to operate in areas with high ambient light levels or direct sunlight. Thus, the range and applicability of prior art photodiode-based, battery-powered optical data transmission systems and remote controls have been limited.
It is highly desirable to provide an optical communications system or remote control that has a range greater than 30 feet and that is not seriously affected by the presence of high ambient light levels. There are many applications that would benefit from such a system, if it were not for the above-described limitations inherent in the prior art. For instance, outdoor systems such as spas or swimming pools would realize distinct advantages from optical remote control systems having a range of 100 feet or more. It is more convenient for a user to adjust water temperature, pumping velocity, pool lighting or the other auxiliary functions without having to exit the spa or pool. Present systems employing direct electrical connection or pneumatic controls demand that the transmitter be mounted at a fixed point relative to the pool or spa. These systems are expensive to install because the lines connecting the control to the equipment must be protected. Pneumatic systems provide only a limited number of control options and electrical systems require elaborate safeguards to minimize the risk of electrical shock arising from the user's direct contact with house-current-powered equipment. Systems employing radio-frequency ("RF") links are expensive and must meet rigorous standards as to construction and operation to prevent interference with other RF and electrical systems. The same arguments can be made for garage door openers, security systems, surveying equipment and even line-of-sight voice communications systems.
One solution to the range and ambient light problem would be to use a solar cell as the signal detector in the receiver. Solar cells, by virtue of their large detector area, are much more sensitive to optical signals of low power. Furthermore, when operating in a "current" or "short circuit" mode, they do not saturate in the presence of strong ambient light. Properly applied, the use of a solar cell as the signal detector could simplify a remote control system, significantly increase its operating range and greatly reduce its sensitivity to ambient light.
Despite the advantages solar cells have over photodiodes, they have not been employed as signal detectors because they are capacitive by nature, exhibiting a capacitance of approximately 0.1 .mu.F per square inch of detector area. The resulting capacitive reactance acts as a low impedance load across the solar cell output. At any useful carrier frequency, this reactive load absorbs a very significant percentage of the received signal energy.
Therefore, what is needed in the art are a circuit and method for allowing remote control from a greater range. Preferably, the circuit and method should take advantage of the sensitivity of a solar cell to weak optical signals but should overcome the concomitant signal-depletion problems associated with such cells.